Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 241347

Area Forecast Discussion
National Weather Service Honolulu HI
347 AM HST Mon Jul 24 2017

A trough of low pressure, the remnants of Fernanda, will be moving
east to west across Oahu and Kauai today. The trough will
interrupt our trades statewide with muggy conditions and the
possibility of heavy showers or thunderstorms. More settled
conditions should start to return tonight as drier air and more
stable air return. Breezy trades, more sunshine, and less humid
conditions are expected for the rest of the week as high pressure
well to the north of the state regains control of our weather.


A surface trough, the remnants of former Hurricane Fernanda, is
moving across the islands this morning. Satellite-derived low
level wind fields indicate the trough axis was over Maui county
during the predawn hours, moving toward the W at about 13 kt.
Winds ahead of the trough are breezy trades, but the background
flow transitions to breezy S to SE behind the trough. The 12z
soundings show a moist and unstable airmass over the state,
MLCAPE ranging from about 800 J/kg at Lihue to 1200 J/kg at Hilo,
with even higher instability likely present between the sounding
sites. This moisture and instability are interacting with an
upper trough located just the W and NW of the islands, and
scattered showers and thunderstorms have blossomed over the last
few hours to the N of Maui Co and E through NE of Oahu.

As the surface trough continues westward today, winds will go
light and variable over Kauai and Oahu, with southerlies becoming
southeasterly elsewhere. Can`t rule out heavy downpours or
thunderstorms moving ashore this morning on Kauai and Oahu, with
the possibility of more heavy showers or thunderstorms blowing up
over the island interiors this afternoon. The mid-level support
is fairly weak and the surface trough is moving right along, so if
heavy showers or thunderstorms do occur, they are not expected to
last too long in any one location.

After today the weather should quiet down fairly quickly, as the
deep tropical moisture and instability associated with ex-
Fernanda departs to the NW. The low level flow will return to
regularly scheduled summertime breezy trades by Tuesday, moisture
will decrease, and the trough aloft will weaken further. Fewer
showers are expected overall beyond today, but some of the models
are showing an modest increase in low level trade wind moisture
Thu night through Fri night. Have increased PoPs a little for this
time period. A sprawling, persistent area of high pressure to the
north of the islands will keep breezy trades blowing locally into
the weekend.


The remnant low associated with former Tropical Cyclone Fernanda
continues to move through the Hawaiian Islands this morning.
Expect scattered to numerous showers through the morning hours
along with scattered MVFR conditions mainly along northeastern
slopes of all islands. Isolated thunderstorms will develop around
the remnant low center near Molokai, Oahu and Kauai.

An AIRMET for IFR conditions for Molokai and an AIRMET for Tempo
Mountain Obscuration in effect for Lanai and the north and eastern
slopes of Molokai and Oahu. This AIRMET may need to be expanded
to include Kauai later this morning as the low ceilings and
showers spread westward.


High pressure centered far northeast of the islands will support
trade winds through the forecast period. A surface trough will
pass east to west over the island chain through today, which will
cause the trade winds to temporarily weaken. Stronger trade winds
will begin filling in across the eastern end of the state as
early as later today. The trade winds will strengthen to Small
Craft Advisory levels by Tuesday morning for the typically windy
zones around Maui County and the Big Island.

One final large high tide of the month will occur tonight. This
tide may result in coastal flooding in some areas. A Special
Weather Statement (see SPSHFO product on our website) is posted to
highlight the potential hazards.

Moisture associated with the incoming surface trough will interact
with a broad upper air feature near the islands through the day.
This interaction could result in isolated thunderstorms over
coastal and offshore waters through tonight (and possibly through
Tuesday for the western offshore waters).




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